1. Field of the Invention
The present invention generally relates to a metal/ceramic circuit board having a ceramic substrate and a metal circuit plate which is formed on the ceramic substrate directly or via a brazing filler metal or the like. More specifically, the invention relates to a metal/ceramic circuit board on which parts, such as semiconductor parts, are mounted and which is used for a power module or a Peltier element module.
2. Description of the Prior Art
In a typical method for producing a ceramic circuit board for a power module or for mounting a semiconductor thereon, a metal plate and a ceramic substrate are first bonded to each other. For example, there is industrially utilized the direct bonding method for arranging a copper plate on a ceramic substrate so as to allow the copper plate to directly contact the ceramic substrate and for heating the copper plate and the ceramic substrate in an inert gas to bond the ceramic substrate and the copper plate to each other. There is also industrially utilized the brazing and soldering method for arranging a copper plate on a ceramic substrate via a brazing filler metal containing an active metal, such as Ti, Zr or Hf, and for heating them in a vacuum to bond the ceramic substrate and the copper plate to each other. In the brazing and soldering method, the active metal concerns the bonding of the ceramic substrate to the metal plate, and the ceramic substrate reacts with the brazing filler metal to form a reaction product.
In circuit boards for power boards, the minimum distance between patterns is generally about 0.5 mm, and it is required to ensure a certain withstand voltage. On the other hand, in circuit boards for Peltier elements and high frequency circuits, it is required to form finer patterns. Therefore, the above described metal plate bonding method is not suitable for the production of such circuit boards, so that a metal film is formed on a ceramic substrate by plating or spattering.
In general, in a case where patterns are formed in a metal/ceramic circuit board by etching, the dimensional difference between the bottom and top faces of the peripheral edge portion of a metal circuit plate is about 50% of the thickness of the metal circuit plate. This difference is the distance between a plane perpendicular to the principal plane of a metal plate at one end of the bottom face of the metal plate and a plane perpendicular to the principal plane of the metal plate at one end of the top face of the metal plate on the same side as the one end of the bottom face of the metal plate, i.e., the length shown by L1 in FIG. 6. It is assumed that the sign of the distance is positive (+) when the area of the bottom face is greater than the area of the top face. This distance will be hereinafter referred to as a “skirt spreading length”. Furthermore, in FIG. 6, L2 denotes a “brazing filler metal protruding length”. For example, in a printed circuit board wherein a copper foil having a thickness of about 10 to 30 μm is applied on a resin substrate, such as a glass epoxy substrate, to form a circuit, or in a circuit board wherein a metal thin film having a thickness of several micrometers is formed on a ceramic substrate by spattering or plating to form a circuit, the skirt spreading lengths are over ten micrometers and several micrometers, respectively. Such dimensional differences do not influence the formation of the circuit and precision. On the other hand, in a substrate where a metal plate is bonded to a ceramic substrate directly or via a brazing filler metal, the metal plate usually has a relatively large thickness of about 100 to 500 μm, and the skirt spreading length of the metal circuit plate is at least in the range of from about 50 μm to about 250 μm. Such a dimensional difference can not ignore in order to form a fine circuit.
In addition, although the skirt spreading length of a metal circuit plate in a conventional metal/ceramic substrate is generally in the range of from about 50 μm to about 200 μm, there has been proposed a metal/ceramic circuit board wherein the skirt spreading length of a metal circuit plate is in the range of from 50 μm to 100 μm (see Japanese Patent Laid-Open No. 10-326949).
However, the demands for compactization in recent years are also made in the field of module. If the area for mounting parts, such as Si chips, is small, the parts are often displaced during the mounting of the parts to protrude part of the parts from the substrate. If this protrusion is extremely large, the parts are displaced to adjacent metal circuit patterns, so that the circuit can not be formed.
In addition, the parts are mounted using a solder paste or the like. If the semiconductor mounting design area (the area of the top of the metal plate) is small, even if the amount of the paste slightly decreases, the possibility of producing voids increases.
Moreover, the discrepancy in mounting of these parts decreases the effective heat transfer area of parts, such as an area below a chip, in a substrate similar to a power module. Therefore, there are some cases where heat dissipation decreases to accumulate heat in the circuit to promote the deterioration of reliability. For example, if a 11×11 mm part is displaced by 0.1 mm in horizontal (x, y) directions in the mounting portion to protrude, respectively, the effective heat transfer area decreases by about 2%, and if a 0.8×1.8 mm part is displace by 0.1 mm in horizontal (x, y) directions in the mounting portion to protrude, respectively, the effective heat transfer area decreases by about 17%. These values are very large values with respect to the radiation design of the substrate. Moreover, if voids are also produced, circumstances have a tendency to further deteriorate. For that reason, the requirements for dimension for mounting parts on the substrate, and the requirements for improvement of costs of defectives are very severe in the circumstance.